consider x being Element of X;
set f = (3 -tuples_on X) --> x;
take (3 -tuples_on X) --> x ; :: thesis: (3 -tuples_on X) --> x is ternary
thus arity ((3 -tuples_on X) --> x) = 3 by Th2; :: according to COMPUT_1:def 27 :: thesis: verum